The present invention relates to a semiconductor device having an improved structure of multi-wiring layers which prevents short-circuiting caused by electromigration.
In recent years, integration density of semiconductor devices such as IC's, LSI's and the like has increased, resulting in complex wiring patterns for interconnecting the elements. Therefore, the use of multi-wiring layers has also increased. The multi-wiring layers consist of a plurality of wiring layers via an intermediate insulating film, and the wiring layers are connected to each other and/or to predetermined parts of a semiconductor substrate through contact holes formed in the insulating film. The material of the wiring layer is mostly aluminum (Al), and the intermediate insulating film is made of silicon oxide (SiO.sub.2), phosphorus silicate glass (PSG), silicon nitride (Si.sub.x N.sub.y), polyimide-type resin, or the like.
Aluminum has good electrical and thermal conductivities and adheres well to the semiconductor substrate or the insulating film such as SiO.sub.2, and is easily patterned. Therefore, aluminum is widely used as a wiring material. However, the aluminum wiring layer often causes electromigration when the semiconductor device is operating with a large current density of 2.times.10.sup.5 A/cm.sup.2 to 5.times.10.sup.5 A/cm.sup.2. Electromigration is a phenomenon in which atoms migrate owing to interraction between the electrons and the atoms constituting the wiring. Progress of this phenomenon in aluminum wiring gives rise to the development of hillocks or crystalline whiskers consisting of aluminum atoms. The whiskers or hillocks from one wiring layer grow penetrating through the intermediate insulating film having a reduced thickness or through portions having small mechanical strength where the density may be relatively small, and reach another aluminum wiring layer positioned above or under the one wiring layer, resulting in short-circuiting between the two wiring layers. To restrain electromigration, copper has been added in small amounts to aluminum, or heat-treatment has been tried so as to increase the size of aluminum crystals, i.e., to decrease the density of grain boundaries, so that fewer aluminum atoms would migrate. These techniques have not given satisfactory results.